1. Field of the Invention
This invention relates to a polyester polymerization catalyst, polyester produced by using the same and a process for producing polyester and in particular to an antimony compound-free novel polyester polymerization catalyst, polyester produced by using the same and a process for producing polyester.
2. Prior Art
Polyester, particularly polyethylene terephthalate (abbreviated hereinafter into PET), is superior in mechanical and chemical properties and applied to various uses such as fibers for clothes and industrial materials, various films such as packaging films and magnetic tapes and sheets, and molded articles such as bottles and engineering plastics.
PET is obtained industrially by esterification or transesterification of terephthalic acid or dimethyl terephthalate with ethylene glycol thereof to produce bis(2-hydroxyethyl) terephthalate and subsequent polycondensation thereof with a catalyst at high temperature in vacuo. As the catalyst used in this polycondensation, antimony trioxide is widely used. Antimony trioxide is a catalyst which is inexpensive and superior in the catalytic activity, but since an antimony metal is precipitated during polycondensation, there is the problem of gray discoloration and formation of insoluble particles in PET. Further, a problem with the safety of antimony is recently noted from an environmental viewpoint. Under these circumstances, there is demand for polyester containing no or little antimony.
While using antimony trioxide as the polycondensation catalyst, an attempt at inhibiting occurrence of gray discoloration and formation of insoluble particles in PET has been made. In Japanese Patent No. 2666502, for example, antimony trioxide and bismuth and selenium compounds are used as the polycondensation catalyst, whereby formation of gray insoluble particles in PET is inhibited. Further, Japanese Unexamined Patent Publication No. 291141/1997 describes that when antimony trioxide containing sodium and iron oxides is used as the polycondensation catalyst, precipitation of an antimony metal is inhibited. However, these polycondensation catalysts cannot achieve the object of obtaining polyester containing no or little antimony.
A polycondensation catalyst substituted for antimony trioxide has also been investigated. In particular, a titanium compound represented by tetraalkoxy titanate has already been proposed, but there is the problem that PET produced using this compound is significantly discolored and easily thermally degraded.
As an attempt to overcome such problem when tetraalkoxy titanate is used as the polycondensation catalyst, for example Japanese Unexamined Patent Publication No. 116722/1980 has proposed a method of using tetraalkoxy titanate with a cobalt salt and a calcium salt. Further, Japanese Unexamined Patent Publication No. 73581/1996 has proposed a method of using tetraalkoxy titanate simultaneously with a cobalt compound as the polycondensation catalyst and simultaneously using a optical brightener. By these proposes, the discoloration of PET is reduced when tetraalkoxy titanate is used as the polycondensation catalyst, but effective inhibition of thermal degradation of PET has not been achieved.
As a polycondensation catalyst which is substituted for antimony trioxide and overcomes such problem when tetraalkoxy titanate is used, a germanium compound has been practically used, but this catalyst has the problem that it is very expensive and that the catalyst easily evaporates from the reaction system to the outside so that the concentration of the catalyst in the reaction system is changed thus making control of polymerization difficult.
It is known that aluminum compounds are generally inferior in the catalytic activity. Among the aluminum compounds, aluminum chelate compounds are reported to have a higher catalytic activity as a polycondensation catalyst than other aluminum compounds, but the catalytic activity is not practically sufficient as compared with the antimony compounds and titanium compounds described above.
This invention provides a novel polycondensation catalyst other than antimony compounds, polyester produced by using the same and a process for producing polyester.